Multifield Ultralight Dark Matter

TL;DR

This paper investigates a multifield ultralight dark matter model with N scalar fields, showing that multiple fields lead to smoother halos and weaker observational constraints compared to single-field models, with specific scaling behaviors for velocity dispersion.

Contribution

It introduces a multifield ULDM framework, analyzes its effects on halo structure and stellar velocity dispersion, and derives scaling laws for observational constraints.

Findings

Density fluctuations decrease as 1/√N with more fields

Smoother halos result in less stellar heating and weaker constraints

Velocity dispersion scales as 1/(N m^3) for equal-mass fields

Abstract

Ultralight dark matter (ULDM) is usually taken to be a single scalar field. Here we explore the possibility that ULDM consists of $N$ light scalar fields with only gravitational interactions. This configuration is more consistent with the underlying particle physics motivations for these scenarios than a single ultralight field. ULDM halos have a characteristic granular structure that increases stellar velocity dispersion and can be used as observational constraints on ULDM models. In multifield simulations, we find that inside a halo the amplitude of the total density fluctuations decreases as $1/\sqrt{N}$ and that the fields do not become significantly correlated over cosmological timescales. Smoother halos heat stellar orbits less efficiently, reducing the velocity dispersion relative to the single field case and thus weakening the observational constraints on the field mass. Analytically, we show that for $N$ equal-mass fields with mass $m$ the ULDM contribution to the stellar velocity dispersion scales as $1/(N m^3)$. Lighter fields heat the most efficiently and if the smallest mass $m_L$ is significantly below the other field masses the dispersion scales as $1/(N^2 m_L^3)$.